Gabriel Weinberg noticed web traffic building on the night of Thursday 6 June – immediately after the revelations about the "Prism" programme. Through the programme, the US's National Security Agency claimed to have "direct access" to the servers of companies including, crucially, the web's biggest search engines – Google, Microsoft and Yahoo.

"It happened with the release by the Guardian about Prism," says Weinberg, right, a 33-year-old living in Paoli, a suburb of Philadelphia on the US east coast. "We started seeing an increase right when the story broke, before we were covered in the press." From serving 1.7m searches a day at the start of June, it hit 3m within a fortnight.

Yet you've probably never heard of DuckDuckGo. "If you asked 100 people, 96 would probably think it was a Chinese restaurant," as the SFGate site observed. (The name comes from the children's game DuckDuckGoose, a sort of tag involving seated players.) You won't find it offered as an alternative default search engine on any browser, on desktop or mobile. Using it is very definitely an active choice, whereas using Google is the default option on most browsers. And 95% of people never change the default settings on anything.

But this 20-person business offers what none of the big search engines do: zero tracking. It doesn't use cookies or store data about its users' IP addresses, doesn't offer user logins, and uses an encrypted connection by default. (Google provides an encrypted connection for logged-in users, but not automatically for non-logged in users.) If the NSA demanded data from DuckDuckGo, there would be none to hand over.

Weinberg, who lives with his wife and two sons, did not build his search engine with that intention. The initial idea came after selling his previous startup, Opobox ("a sort of Friends Reunited"), for $10m (£6.7m) to Classmates.com in 2006. "My wife was doing her PhD, so I had some spare time," he says. Taking a class in stained-glass making, he discovered that the teacher's handout with "useful web links" didn't tally with Google's results at all. "I realised that there were millions of people who knew the right list of search terms and would make a better engine than Google."

Then he noticed growing amounts of junk sites in Google results – pushed there by experts who had gamed the giant's algorithms. He decided that by hooking into web services such as Wikipedia, Yelp and Qype, he could get focused answers cheaply. By using a combination of those services and crowdsourced links, he built the site's first search index.

Of the privacy angle, he says: "I kind of backed into that." It wasn't a political decision, but a personal one. "It's hard to define my politics. I take every issue seriously and come to my own conclusion. I don't really feel like I belong to any political party in the US … I guess I'm more on the liberal side."

The reason he decided not to store search data was because it reveals so much about us. In 2005, AOL accidentally released details of searches made by 650,000 of its users via Google; reporters from the New York Times were able to use the information to identify one of the users: a 62-year-old woman in Georgia. Nowadays Google would also have your IP address (indicating your ISP and perhaps precise location) and, if you were logged in, all your previous search history. If you logged in to use Google on your mobile, it would have your location history too.

Having decided that searching is intimately personal, he deduced that governments would want to get hold of search data. "I looked at the search fiascos such as the AOL data release, and decided that government requests were real and would be inevitable, and that search engines and content companies would be handing over that data [to government] in increasing amounts."

Search data, he says, "is arguably the most personal data people are entering into anything. You're typing in your problems, your desires. It's not the same as things you post publicly on a social network."

So why does Google store it? "It's a myth that Google needs to store all this data about you. Almost all the money they make on search is based on what you type into the search box. Nothing more. They need to track you for their other services – Gmail, YouTube – because those are hard to monetise, and that's why you get ads following around the internet all the time." (Google owns DoubleClick, the largest display ad supplier online.)

Photograph: guardian.co.uk

Having your data passed around can also lead you to be charged more for an item: if your browsing history shows you visit high-end sites, some sites will increase prices. (That's why plane fares can drop if you delete the "cookie" files in your browser.)

Google's mis-steps are turning out to be DuckDuckGo's biggest source of new users. In January 2012 – when Google announced that it would be aggregating user data across all its services - DuckDuckGo's traffic (which it publishes online) trebled in three months. Once Google implemented the change, "people came and stayed; it wasn't just a rise and fall," Weinberg says.

More recently, the Prism fallout has seen traffic keep rising, building on that success. "I think these people are going to stay too."

He wasn't that surprised at the Prism revelations. "A few months ago 60 Minutes did a programme about this humungous data centre the NSA is building in Utah. After hearing that, this didn't surprise me that much. But it did surprise me how much we have increased our traffic."

Even so, not everyone believes Weinberg's success matters much. Danny Sullivan, who runs the Search Engine Land site, and has been analysing the search business since Google was just a gleam in the eyes of Larry Page and Sergey Brin, argues that DuckDuckGo's size really indicates people don't care about privacy.

"Don't get me wrong. If you ask people about search privacy, they'll respond that it's a major issue," he wrote on his site. "Big majorities say they don't want to be tracked nor receive personalised results. But if you look at what people actually do, virtually none of them make efforts to have more private search." Compared with the 13bn searches Google does every day, he suggests, DuckDuckGo's 3m daily (90m monthly) barely registers.

Is that because people don't know it exists? Is it like Google in 1998, when the dominant search engine was Altavista (closed this week by Yahoo)? "I don't think that's it," Sullivan said. "Ask.com was pretty well-known. It did a big privacy push; didn't help. Yahoo played up [privacy] against Google; nope. I think most people trust Google – enough, at least."